Somatic cell hybridization will be used to investigate the molecular mechanisms regulating cellular aging, disorders of premature senescence and myogenic cell differentiation. Heterokaryons and cell hybrids will be isolated using a technique we have developed that does not require cells with heritable seletive enzyme dificiencies. We cah thus examine the behavior of fusion products between normal diploid cells. The cultured lifespan of cell hybrids between young and old human fibroblasts will indicate the dominance or recessiveness of the senescent phenotype. The potential for genetic complementation will be examined in hybrids between cells from patients with progeric syndromes. The contribution of chromosome 6 to this process will be determined by using antibodies against specific HLA alleles to select for hybrids that have eliminated the parental chromosome carrying that allele. Alterations in hybrid lifespan following the elimination of chromosome 6 from one parent will implicate genes on that chromosome, particularly a major histocompatibility complex linked "supergene,' in the regulation of cellular senescence. HLA abnormalities have been observed in fibroblasts from patients with progeroid syndromes. The same approach will be used to assess the importance of chromosome 6 to the lifespan of hybrid cells between normal and progeric fibroblasts. We have shown that factors provided by differentiated chick myocytes can induce rat myosin synthesis when these cells are fused to undifferentiated rat myoblasts. Fusions between anucleated chick myocyte cytoplasms and undifferentiated rat myoblasts will indicate if the factor is nuclear or cytoplasmic. The red cell mediated microinjection nof appropriate cell fractions may then permit us to isolate the inducing molecules. The analysis of heterokaryons formed by fusing chick myocytes to a variety of other cells (BudR-blocked rat myoblasts, rat fibroblasts, primitive teratocarcinoma cells, cardiac myocytes) will probe other regulatory interactions during development. These studies will generate information leading to a better understanding of the molecular mechanisms regulating normal and pathologic processes during early (cell differentiation) and late (cellular senescence) development.